Among third-generation mobile communication systems which are examples of radio communication systems, a W-CDMA (Wideband-Code Division Multiple Access) system is adopted. The radio interface in the W-CDMA system is constituted of protocol layers of physical layer (Layer 1), data link layer (Layer 2) and network layer (Layer 3), and Layer 2 (L2) is divided into sublayers of MAC (Media Access Control) and RLC (Radio Access Control). Data channel (DCH) allocation of a packet data in packet transmission is carried out in the L2 RLC sublayer, and the data format is defined in a 3GPP specification (TS25.322) as RLC SDU (Service Data unit).
In the above RLC SDU, there are a few PDU (Protocol Data Unit) formats according to transfer modes, being defined such as TrD PDU (Transparent Mode), UMD PDU (Unacknowledge Mode) and AMD PDU (Acknowledge Mode), among which header information, such as SN (Sequence Number) and LI (Length Indicator), is added in case of UMD PDU and AMD PDU.
FIG. 1 shows a diagram illustrating the UMD PDU data format. The data format concerned includes a header part, including SN (Sequence Number) and LI (Length Indicator), and a data part. In the data part, data are stored and further, padding (PAD) may be added thereto.
The LI information in the header part is information for indicating the connection portion to a next packet (how many octets are present in the PDU before a breakpoint) when dividing a packet to fit to a PDU size at the time of transmitting the packet data by use of PDU, and the packet is divided across PDUs. Accordingly, when the packet is divided to fit to the PDU size, the LI information, which indicates the boundary position between the tail end of the packet concerned and the top of the next packet, is included in the header part of the PDU in which the tail end of the packet is stored. Further, there may also be a case that a plurality of sets of LI information are included in a single PDU when the packet length is shorter than the PDU size.
Additionally, in addition to the case of indicating the boundary position between packets, the LI information is also used when padding is added (the boundary position between the packet data and the padding) or when indicating such a PDU storage condition that the packet connection portion comes to the final PDU octets, and is specified as a special code.
Further, the Japanese Unexamined Patent Publication No. 2000-125004, No. 2003-92600 and the Japanese Patent Publication No. 11-298534 respectively relate to PDU transmission using a format having LI information.